AIRL

AIRL, NRB, RIGID_CALC
Specifies that automatic inertia relief calculations are to be performed.

SOLUTION: Inertia

NRB

Number of rigid body modes in the model:

AUTO	—	Activate automatic inertia relief (default). The program automatically determines the number of rigid body modes.
*`n`*	—	Activate automatic inertia relief with the assumption of `n` rigid body modes (1≤`n` ≤ 6).
0	—	Deactivate automatic inertia relief.

RIGID_CALC

Method for computing the rigid body modes:

0	—	Use an eigensolver to compute rigid body modes (default).
1	—	Use the geometry to compute rigid body modes (valid only when `NRB` = AUTO). The geometric rigid body vectors are calculated according to Equation 15–173 in Participation Factors and Mode Coefficients in the Theory Reference.

Command Default

No automatic inertia relief calculations.

Notes

The AIRL command activates automatic inertia relief for models having up to six rigid body modes. This method is only valid for the following analysis types:

linear static analysis (ANTYPE,STATIC)
modal analysis (ANTYPE,MODAL) when generating residual vectors (RESVEC)

By default (NRB = AUTO), the number of rigid body modes in the structure is automatically determined. The static solution is altered so that the inertial effects factor into counterbalancing the external loads. This method relies on calculation of the rigid body modes using an eigensolver (RIGID_CALC = 0, which is the default). When running shared-memory parallel (SMP), it is also possible to enable the calculation of the rigid body modes using the model geometry (RIGID_CALC = 1).

For the NRB = AUTO option, no supports should be defined.

For a model that is partially constrained by design, you must set NRB to the number of rigid body modes present in the structure and set RIGID_CALC = 0. The use of geometry (RIGID_CALC = 1) to compute the rigid-body modes of a partially constrained model is not supported.

Loads may be input as usual. Displacements and stresses are calculated as usual.

Use IRLIST to print inertia relief calculation results. The mass and moment of inertia summary printed before the solution is accurate (because of the additional pre-calculations required for inertia relief). See Inertia Relief in the Theory Reference for calculation details. See Including Inertia Relief Calculations in the Basic Analysis Guide for procedural details, including requirements and limitations.

When RIGID_CALC = 0, inertia relief calculation results are available only when the node chosen by the algorithm as the virtual reference point (mentioned in Linear Static Analysis Without Supports) has six degrees of freedom.

Inertia relief accelerations are not available when running distributed-memory parallel (DMP).

The program supports generating residual vectors using AIRL only for models with a symmetric stiffness matrix.

This command is also valid in PREP7.

Example Usage

Example command input for including automatic inertia relief calculations in a linear static analysis is found in Including Inertia Relief Calculations in the Basic Analysis Guide.

Example command input for including automatic inertia relief for generating residual vector of an unconstrained model is found in Command Input to Include Automatic Inertia Relief for Calculation of the Residual Vectors in the Modal Analysis for Unconstrained Models in the Basic Analysis Guide

Menu Paths

This command cannot be accessed from a menu.